$-4su + 7t + 9u + 2 = 6t + 4u - 9$ Solve for $s$.
Solution: Combine constant terms on the right. $-4su + 7t + 9u + {2} = 6t + 4u - {9}$ $-4su + 7t + 9u = 6t + 4u - {11}$ Combine $u$ terms on the right. $-4su + 7t + {9u} = 6t + {4u} - 11$ $-4su + 7t = 6t - {5u} - 11$ Combine $t$ terms on the right. $-4su + {7t} = {6t} - 5u - 11$ $-4su = -{t} - 5u - 11$ Isolate $s$ $-{4}s{u} = -t - 5u - 11$ $s = \dfrac{ -t - 5u - 11 }{ -{4u} }$ Swap the signs so the denominator isn't negative. $s = \dfrac{ {1}t + {5}u + {11} }{ {4u} }$